目录：
一：timerfd介绍1；
二：timerfd介绍2
三：终结

一.timerfd介绍1：

timerfd是Linux为用户程序提供的一个定时器接口。这个接口基于文件描述符，所以能够被用于select/poll的应用场景。
1. 使用方法
timerfd提供了如下接口供用户使用
timerfd_create()
int timerfd_create(int clockid, int flags);
timerfd_create用于创建一个定时器文件。
参数clockid可以是CLOCK_MONOTONIC或者CLOCK_REALTIME。
参数flags可以是0或者O_CLOEXEC/O_NONBLOCK。
函数返回值是一个文件句柄fd。
timerfd_settime()
int timerfd_settime(int ufd, int flags, const struct itimerspec * utmr, struct itimerspec * otmr);
此函数用于设置新的超时时间，并开始计时。
参数ufd是timerfd_create返回的文件句柄。
参数flags为1代表设置的是绝对时间；为0代表相对时间。
参数utmr为需要设置的时间。
参数otmr为定时器这次设置之前的超时时间。
函数返回0代表设置成功。
timerfd_gettime()
int timerfd_gettime(int ufd, struct itimerspec * otmr);
此函数用于获得定时器距离下次超时还剩下的时间。如果调用时定时器已经到期，并且该定时器处于循环模式（设置超时时间时struct itimerspec::it_interval不为0），那么调用此函数之后定时器重新开始计时。
read()
当timerfd为阻塞方式时，read函数将被阻塞，直到定时器超时。
函数返回值大于0，代表定时器超时；否则，代表没有超时（被信号唤醒，等等）。
poll/close()
poll，close与标准文件操作相同。

man帮助手册：

SYNOPSIS         top

       #include <sys/timerfd.h>

       int timerfd_create(int clockid, int flags);

       int timerfd_settime(int fd, int flags,
                           const struct itimerspec *new_value,
                           struct itimerspec *old_value);

       int timerfd_gettime(int fd, struct itimerspec *curr_value);
DESCRIPTION         top

       These system calls create and operate on a timer that delivers timer
       expiration notifications via a file descriptor.  They provide an
       alternative to the use of setitimer(2) or timer_create(2), with the
       advantage that the file descriptor may be monitored by select(2),
       poll(2), and epoll(7).

       The use of these three system calls is analogous to the use of
       timer_create(2), timer_settime(2), and timer_gettime(2).  (There is
       no analog of timer_getoverrun(2), since that functionality is
       provided by read(2), as described below.)
timerfd_create()

       timerfd_create() creates a new timer object, and returns a file
       descriptor that refers to that timer.  The clockid argument specifies
       the clock that is used to mark the progress of the timer, and must be
       either CLOCK_REALTIME or CLOCK_MONOTONIC.  CLOCK_REALTIME is a
       settable system-wide clock.  CLOCK_MONOTONIC is a nonsettable clock
       that is not affected by discontinuous changes in the system clock
       (e.g., manual changes to system time).  The current value of each of
       these clocks can be retrieved using clock_gettime(2).

       Starting with Linux 2.6.27, the following values may be bitwise ORed
       in flags to change the behavior of timerfd_create():

       TFD_NONBLOCK  Set the O_NONBLOCK file status flag on the new open
                     file description.  Using this flag saves extra calls to
                     fcntl(2) to achieve the same result.

       TFD_CLOEXEC   Set the close-on-exec (FD_CLOEXEC) flag on the new file
                     descriptor.  See the description of the O_CLOEXEC flag
                     in open(2) for reasons why this may be useful.

       In Linux versions up to and including 2.6.26, flags must be specified
       as zero.
timerfd_settime()

       timerfd_settime() arms (starts) or disarms (stops) the timer referred
       to by the file descriptor fd.

       The new_value argument specifies the initial expiration and interval
       for the timer.  The itimer structure used for this argument contains
       two fields, each of which is in turn a structure of type timespec:

           struct timespec {
               time_t tv_sec;                /* Seconds */
               long   tv_nsec;               /* Nanoseconds */
           };

           struct itimerspec {
               struct timespec it_interval;  /* Interval for periodic timer */
               struct timespec it_value;     /* Initial expiration */
           };

       new_value.it_value specifies the initial expiration of the timer, in
       seconds and nanoseconds.  Setting either field of new_value.it_value
       to a nonzero value arms the timer.  Setting both fields of
       new_value.it_value to zero disarms the timer.

       Setting one or both fields of new_value.it_interval to nonzero values
       specifies the period, in seconds and nanoseconds, for repeated timer
       expirations after the initial expiration.  If both fields of
       new_value.it_interval are zero, the timer expires just once, at the
       time specified by new_value.it_value.

       The flags argument is either 0, to start a relative timer
       (new_value.it_value specifies a time relative to the current value of
       the clock specified by clockid), or TFD_TIMER_ABSTIME, to start an
       absolute timer (new_value.it_value specifies an absolute time for the
       clock specified by clockid; that is, the timer will expire when the
       value of that clock reaches the value specified in
       new_value.it_value).

       If the old_value argument is not NULL, then the itimerspec structure
       that it points to is used to return the setting of the timer that was
       current at the time of the call; see the description of
       timerfd_gettime() following.
timerfd_gettime()

       timerfd_gettime() returns, in curr_value, an itimerspec structure
       that contains the current setting of the timer referred to by the
       file descriptor fd.

       The it_value field returns the amount of time until the timer will
       next expire.  If both fields of this structure are zero, then the
       timer is currently disarmed.  This field always contains a relative
       value, regardless of whether the TFD_TIMER_ABSTIME flag was specified
       when setting the timer.

       The it_interval field returns the interval of the timer.  If both
       fields of this structure are zero, then the timer is set to expire
       just once, at the time specified by curr_value.it_value.
Operating on a timer file descriptor

       The file descriptor returned by timerfd_create() supports the
       following operations:

       read(2)
              If the timer has already expired one or more times since its
              settings were last modified using timerfd_settime(), or since
              the last successful read(2), then the buffer given to read(2)
              returns an unsigned 8-byte integer (uint64_t) containing the
              number of expirations that have occurred.  (The returned value
              is in host byte order, i.e., the native byte order for
              integers on the host machine.)

              If no timer expirations have occurred at the time of the
              read(2), then the call either blocks until the next timer
              expiration, or fails with the error EAGAIN if the file
              descriptor has been made nonblocking (via the use of the
              fcntl(2) F_SETFL operation to set the O_NONBLOCK flag).

              A read(2) will fail with the error EINVAL if the size of the
              supplied buffer is less than 8 bytes.

       poll(2), select(2) (and similar)
              The file descriptor is readable (the select(2) readfds
              argument; the poll(2) POLLIN flag) if one or more timer
              expirations have occurred.

              The file descriptor also supports the other file-descriptor
              multiplexing APIs: pselect(2), ppoll(2), and epoll(7).

       close(2)
              When the file descriptor is no longer required it should be
              closed.  When all file descriptors associated with the same
              timer object have been closed, the timer is disarmed and its
              resources are freed by the kernel.
fork(2) semantics

       After a fork(2), the child inherits a copy of the file descriptor
       created by timerfd_create().  The file descriptor refers to the same
       underlying timer object as the corresponding file descriptor in the
       parent, and read(2)s in the child will return information about
       expirations of the timer.
execve(2) semantics

       A file descriptor created by timerfd_create() is preserved across
       execve(2), and continues to generate timer expirations if the timer
       was armed.
RETURN VALUE         top

       On success, timerfd_create() returns a new file descriptor.  On
       error, -1 is returned and errno is set to indicate the error.

       timerfd_settime() and timerfd_gettime() return 0 on success; on error
       they return -1, and set errno to indicate the error.
ERRORS         top

       timerfd_create() can fail with the following errors:

       EINVAL The clockid argument is neither CLOCK_MONOTONIC nor
              CLOCK_REALTIME;

       EINVAL flags is invalid; or, in Linux 2.6.26 or earlier, flags is
              nonzero.

       EMFILE The per-process limit of open file descriptors has been
              reached.

       ENFILE The system-wide limit on the total number of open files has
              been reached.

       ENODEV Could not mount (internal) anonymous inode device.

       ENOMEM There was insufficient kernel memory to create the timer.

       timerfd_settime() and timerfd_gettime() can fail with the following
       errors:

       EBADF  fd is not a valid file descriptor.

       EFAULT new_value, old_value, or curr_value is not valid a pointer.

       EINVAL fd is not a valid timerfd file descriptor.

       timerfd_settime() can also fail with the following errors:

       EINVAL new_value is not properly initialized (one of the tv_nsec
              falls outside the range zero to 999,999,999).

       EINVAL flags is invalid.

示例代码：

The following program creates a timer and then monitors its progress. The program accepts up to three command-line arguments. The first argument specifies the number of seconds for the initial expiration of the timer. The second argument specifies the interval for the timer, in seconds. The third argument specifies the number of times the program should allow the timer to expire before terminating. The second and third command-line arguments are optional. 

#include <sys/timerfd.h>
#include <time.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>        /* Definition of uint64_t */

#define handle_error(msg) \
    do { perror(msg); exit(EXIT_FAILURE); } while (0)

static void
print_elapsed_time(void)
{
    static struct timespec start;
    struct timespec curr;
    static int first_call = 1;
    int secs, nsecs;

    if (first_call) {
        first_call = 0;
        if (clock_gettime(CLOCK_MONOTONIC, &start) == -1)
            handle_error("clock_gettime");
    }

    if (clock_gettime(CLOCK_MONOTONIC, &curr) == -1)
        handle_error("clock_gettime");

    secs = curr.tv_sec - start.tv_sec;
    nsecs = curr.tv_nsec - start.tv_nsec;
    if (nsecs < 0) {
        secs--;
        nsecs += 1000000000;
    }
    printf("%d.%03d: ", secs, (nsecs + 500000) / 1000000);
}

int
main(int argc, char *argv[])
{
    struct itimerspec new_value;
    int max_exp, fd;
    struct timespec now;
    uint64_t exp, tot_exp;
    ssize_t s;

    if ((argc != 2) && (argc != 4)) {
        fprintf(stderr, "%s init-secs [interval-secs max-exp]\n",
                argv[0]);
        exit(EXIT_FAILURE);
    }

    if (clock_gettime(CLOCK_REALTIME, &now) == -1)
        handle_error("clock_gettime");

    /* Create a CLOCK_REALTIME absolute timer with initial
    expiration and interval as specified in command line */

    new_value.it_value.tv_sec = now.tv_sec + atoi(argv[1]);
    new_value.it_value.tv_nsec = now.tv_nsec;
    if (argc == 2) {
        new_value.it_interval.tv_sec = 0;
        max_exp = 1;
    } else {
        new_value.it_interval.tv_sec = atoi(argv[2]);
        max_exp = atoi(argv[3]);
    }
    new_value.it_interval.tv_nsec = 0;

    fd = timerfd_create(CLOCK_REALTIME, 0);
    if (fd == -1)
        handle_error("timerfd_create");

    if (timerfd_settime(fd, TFD_TIMER_ABSTIME, &new_value, NULL) == -1)
        handle_error("timerfd_settime");

    print_elapsed_time();
    printf("timer started\n");

    for (tot_exp = 0; tot_exp < max_exp;) {
        s = read(fd, &exp, sizeof(uint64_t));
        if (s != sizeof(uint64_t))
            handle_error("read");

        tot_exp += exp;
        print_elapsed_time();
        printf("read: %llu; total=%llu\n",
               (unsigned long long) exp,
               (unsigned long long) tot_exp);
    }

    exit(EXIT_SUCCESS);
}

二、timerfd介绍2:

timerfd是Linux为用户程序提供的一个定时器接口。这个接口基于文件描述符，通过文件描述符的可读事件进行超时通知，所以能够被用于select/poll的应用场景。timerfd是linux内核2.6.25版本中加入的借口。
timerfd、eventfd、signalfd配合epoll使用，可以构造出一个零轮询的程序，但程序没有处理的事件时，程序是被阻塞的。这样的话在某些移动设备上程序更省电。

clock_gettime函数可以获取系统时钟，精确到纳秒。需要在编译时指定库：-lrt。可以获取两种类型事件：
CLOCK_REALTIME：真实时间（直译，其实是系统时间），从1970.1.1到目前的时间。更改系统时间会更改获取的值。也就是，它以系统时间为坐标。
CLOCK_MONOTONIC：单调时间，获取的时间为系统重启到现在的时间，更改系统时间对齐没有影响。

timerfd_create：
生成一个定时器对象，返回与之关联的文件描述符。接收两个入参，一个是clockid，填写CLOCK_REALTIME或者CLOCK_MONOTONIC，参数意义同上。第二个可以传递控制标志：TFD_NONBLOCK（非阻塞），TFD_CLOEXEC（同O_CLOEXEC）

注：timerfd的进度要比usleep要高。

timerfd_settime：能够启动和停止定时器；可以设置第二个参数：flags，0表示是相对定时器，TFD_TIMER_ABSTIME表示是绝对定时器。
第三个参数设置超时时间，如果为0则表示停止定时器。定时器设置超时方法：
1、设置超时时间是需要调用clock_gettime获取当前时间，如果是绝对定时器，那么需要获取CLOCK_REALTIME，在加上要超时的时间。如果是相对定时器，要获取CLOCK_MONOTONIC时间。
2、数据结构：
struct timespec {
time_t tv_sec; /* Seconds */
long tv_nsec; /* Nanoseconds */
};
struct itimerspec {
struct timespec it_interval; /* Interval for periodic timer */
struct timespec it_value; /* Initial expiration */
};
it_value是首次超时时间，需要填写从clock_gettime获取的时间，并加上要超时的时间。 it_interval是后续周期性超时时间，是多少时间就填写多少。
注意一个容易犯错的地方：tv_nsec加上去后一定要判断是否超出1000000000（如果超过要秒加一），否则会设置失败。
it_interval不为0则表示是周期性定时器。
it_value和it_interval都为0表示停止定时器。

注：timerfd_create第一个参数和clock_gettime的第一个参数都是CLOCK_REALTIME或者CLOCK_MONOTONIC，timerfd_settime的第二个参数为0（相对定时器）或者TFD_TIMER_ABSTIME，三者的关系：
1、如果timerfd_settime设置为TFD_TIMER_ABSTIME（决对定时器），则后面的时间必须用clock_gettime来获取，获取时设置CLOCK_REALTIME还是CLOCK_MONOTONIC取决于timerfd_create设置的值。
2、如果timerfd_settime设置为0（相对定时器），则后面的时间必须用相对时间，就是：
new_value.it_value.tv_nsec = 500000000;
new_value.it_value.tv_sec = 3;
new_value.it_interval.tv_sec = 0;
new_value.it_interval.tv_nsec = 10000000;

read函数可以读timerfd，读的内容为uint_64，表示超时次数。

timerfd简单的性能测试：
申请1000个定时器，超时间定位1s，每秒超时一次，发现cpu占用率在3.0G的cpu上大概为1%，10000个定时器的话再7%左右，而且不会出现同时超时两个的情况，如果有printf到前台，则一般会出现定时器超时多次（3-5）才回调。

三、总结
1.超时的判断是通过timerfd对应的文件有更新（超时次数）；
2.所以可用epoll机制来监听多个timerfd创建的fd，实现一个进程的多计时器管理；
3.相对定时器还是绝对定时器，并不依赖于你用的是真实时间还是单调时间（待验证）；